Point-of-use fluid regulating system for use in the chemical-mechanical planarization of semiconductor wafers

ABSTRACT

The present invention is directed to an apparatus and method for flow regulation of planarization fluids to a semiconductor wafer planarization machine. In one embodiment, the regulating system includes a fluid storage tank with an acoustic fluid level sensor. The storage tank is connected to a fluid delivery line that delivers planarization fluid to the storage tank through a flow control valve and delivers a regulated flow of planarization fluid to a planarization machine through a flow sensor. A gas supply system is connected to the storage tank to provide system pressurization. Regulation of the fluid flow is achieved by a control system in which the flow sensor and the acoustic fluid level sensor comprise feedback elements in a closed feedback system to independently control the pressure in the storage tank and the fluid admitted by the control valve. In an alternate embodiment, the fluid level sensor is comprised of capacitive proximity sensors located outside the wall of the storage tank. In another embodiment, the fluid level sensor is replaced by a buoyant float that can seat in the upper or lower ends of the storage tank and a differential pressure sensor that senses differences in storage tank pressure when the float is seated in either of these locations to indicate full or empty tank conditions. In still another aspect, two or more regulators may be joined in a parallel flow arrangement in order to achieve precise point-of-use mixing of multi-component planarization fluids.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Patent Application No.09/691,805, U.S. Pat. No. 6,431,950 filed Oct. 18, 2000.

TECHNICAL FIELD

This invention relates to chemical-mechanical planarization ofsemiconductor wafers, and more particularly to fluid flow regulatingsystems used in such machines.

BACKGROUND OF THE INVENTION

As the level of integration increases on semiconductor wafers, surfaceirregularities on the wafer have become a serious problem. For example,metallization layers used to form interconnects between the variousdevices on the wafer may lead to substantial surface irregularities thatinterfere with the performance of subsequent photolithographic steps onthe wafer. In order to flatten these surface irregularities, numerousmaterials or methods have been developed, such as SOG (Spin on Glass),and reflow. However, since these methods cannot globally planarize thewafer surface and may not sufficiently remove wafer surfaceirregularities, they have largely given way to the use of polishingtechniques to planarize the surface of semiconductor wafers.

In one commonly used technique, known as chemical-mechanicalplanarization, the semiconductor wafer is mounted in a wafer carrier,and a polishing pad is held on a platen that can be rotated. The exposedsurface of the wafer is then pressed against the polishing pad with aprescribed down force, and the polishing pad and/or the wafer are thenindependently rotated while the wafer carrier is translated across thepad surface. The process is continued until the desired degree ofsurface uniformity on the wafer is attained. In this technique, theabrasive mechanism is generally provided by a planarization fluid thatcontains abrasive particles in suspension with a combination of chemicaletchants that are formulated to etch and dissolve certain materials thatcomprise the wafer. Alternatively, the planarization fluid may containonly the chemical etchants, with the abrasive elements embedded in a“fixed abrasive” pad.

The planarization fluids used in chemical-mechanical planarization aremost commonly supplied to wafer manufacturers in a commerciallyprepackaged form, which may be comprised of two or more parts that arecombined prior to planarizing a production run of wafers. Once thecomponents are mixed, the planarization fluid is distributed to waferplanarization machines by a planarization fluid distribution system.Numerous disadvantages are present in planarization fluid distributionsystems which are explained more fully with reference to the structureand operation of a typical prior art planarization fluid distributionsystem 10 which is shown in FIG. 1.

With reference now to FIG. 1, carefully measured volumes ofplanarization fluid components 130 and 132 are combined in a mixing tank138 to form a planarization fluid 14. The mixing tank 138 has amechanical agitator 136 that is driven by an electric motor 134 to mixthe components and to keep the abrasive component of planarization fluid14 in suspension. After the planarization fluid 14 has been sufficientlymixed, the planarization fluid 14 is transferred to a storage tank 12through line 120. The storage tank 12 has an outlet pipe 18 fortransferring planarization fluid 14 from the tank 12 to a planarizationfluid distribution loop 140. A peristaltic pump 124 that is driven by amotor 122 pumps planarization fluid around the distribution loop 140.Planarization fluid distribution branches 160 a-160 d allowplanarization fluid 14 to be distributed to planarizing machines 126a-126 d, and the amount of planarization fluid 14 distributed to themachines 126 a-126 d may be controlled by manually actuated valves 150a-150 d. Although only four planarization machines are shown for clarityof presentation, a larger number of machines may be present in a typicalsystem. By maintaining constant fluid motion in the distribution loop140, abrasive settling in the distribution loop 140 is avoided.Moreover, the constant pumping of planarization fluid 14 from storagetank 12 to the distribution loop 140, together with the return of theunused portion of the planarization fluid 14 to the storage tank 12through return pipe 16 may keep the abrasive components of planarizationfluid 14 sufficiently agitated.

One disadvantage of the prior art fluid distribution system 10 is thatit will not permit planarization fluids to be mixed from constituentcomponents close to the machine. The mixing and use of planarizationfluid on an as-needed basis is advantageous because the chemicaletchants present in the fluid are subject to chemical degradation, andshould be used relatively soon after mixing occurs. The combination offluid components at the machine will generally permit smaller volumes tobe mixed which may be more completely consumed in the wafer planarizingprocess, thus minimizing the waste of planarization fluid.

Another disadvantage of the prior art distribution system 10 is that itcannot accurately regulate the amount of planarization fluid deliveredto each machine. Referring again to FIG. 1, a peristaltic pump 124 isused to deliver the planarization fluid 14 to the machines 126 a-126 d.Since the peristaltic pump 124 is sensitive to changes in the fluidlevel in the tank 12, the amount of fluid delivered to machines 126a-126 d will vary as the planarization fluid 14 is used. Consequently,the delivery of planarization fluid to machines 126 a-126 d in uniform,precisely regulated amounts cannot be readily accomplished in the priorart system 10.

Still other problems are inherent in the prior art planarization fluiddistribution system 10. For example, the prior art planarization fluiddistribution system 10 requires a minimum volume of planarization fluid14 in order to operate, and depending on the size of the system, thisvolume may be considerable. With reference again to FIG. 1, it is seenthat the planarization fluid distribution system 10 requires that thedistribution loop 140 be filled with planarization fluid 14 duringoperation, and that the storage tank 12 contain a sufficient volume ofplanarization fluid to permit pumping from the storage tank 12.Consequently, when all wafer planarization processing is completed, asignificant volume of unused planarization fluid is retained within thesystem 10. Since the unused planarization fluid loses its effectivenessover time, it cannot be retained for use in planarizing subsequent waferproduction runs and is generally discarded. This waste contributes tothe overall cost to produce the wafer since commercially availableplanarization fluid formulations are relatively costly. Still othercosts are incurred in discarding the excess planarization fluid, becauseit must be disposed of as toxic waste.

Still other disadvantages are associated with the prior artplanarization fluid distribution system 10. For example, after theremoval and disposal of the excess planarization fluid, the entiredistribution system is flushed with deionized water to remove theremaining fluid. However, flushing the distribution system presentsstill other waste disposal problems since the water used to flush thesystem generally contains significant concentrations of chemicalconstituents, as well as abrasives. It must therefore be processed toremove these materials before the water can be discharged into amunicipal wastewater disposal system. An additional problem associatedwith flushing the system is that there is usually no way to remove thede-ionized water that is retained in the distribution system after it isflushed and drained. If the distribution system has a significantvolume, considerable amounts of water will remain in the system afterflushing. Consequently, the water retained by the system will dilute thefresh planarization fluid mixture when it is transported through thesystem. This diluted planarization fluid may cause inconsistentplanarization results in subsequent wafer production runs.

Finally, abrasive settling problems are not effectively addressed by theprior art planarization fluid distribution system 10. Abrasive settling,in particular, is a significant problem in wafer planarization becauseabrasive-rich mixtures generally form in regions near the bottom ofstorage vessels, mixing tanks and distribution lines. Once formed, thesemixtures may lead to uneven planarization of the wafer, or cause thewafer to be planarized beyond the desired endpoint. Moreover, if theabrasive settling is not controlled, large agglomerations of abrasiveparticles may ultimately form in the planarization fluid that may leadto surface scratching of the wafer. Although the prior art distributionsystem 10 uses a distribution loop 140 to inhibit abrasive settling,abrasive particles may still settle in locations that are not subject torecirculation. For example, since wafer planarization generally occurson a periodic basis, the machines must be stopped in order to removeplanarized wafers from the wafer holders and to load unprocessed wafersinto the wafer holders. During this period, the flow of planarizationfluid 14 from the distribution loop 140 to the machines 126 a-126 d isstopped by closing valves 150 a-150 d, which allows the planarizationfluid 14 to remain stationary within the distribution branches 160 a-160d, thus allowing the abrasives to settle and agglomerate. Reestablishingmovement of the planarization fluid in the distribution lines will not,in general, significantly break up these agglomerations once they haveformed.

Many of the shortcomings inherent in prior art planarization fluiddistribution systems could be eliminated if the fluid could be suppliedto the planarization machines individually from a point-of-useplanarization fluid distribution system. As used herein, the term“point-of-use” refers to a fluid distribution system that is located inproximity to the planarization machine that supplies planarization fluidto an individual planarization machine.

Since the point-of-use system is located in proximity to the machine,the need for long distribution lines and recirculation loops iseliminated. Further, since a point-of-use system supplies planarizationfluid to individual planarization machines, the internal volume of thesystem can be small. Consequently, many of the large volume componentsassociated with the prior art planarization fluid distribution systems,such as recirculating loops, large mixing containers and storage tanksare eliminated. As discussed above, the large volume componentscomprising the prior art distribution system are generally recognized assignificant contributors to planarization fluid waste and systemcleaning difficulties.

A point-of-use system capable of precise flow regulation will alsoeliminate planarization fluid flow regulation problems that stem fromthe input pressure sensitivity inherent in peristaltic pumps, therebypermitting a more efficient utilization of planarization fluid. Preciseflow regulation will additionally permit the components of amulti-component planarization fluid to be combined just prior todepositing the mixture on the planarization pad so that fluid issupplied on an as-needed basis, which greatly reduces waste.

Other advantages of the invention will become apparent based upon thedescription of the invention provided below when read with reference tothe drawing figures.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method forplanarization fluid flow regulation that allows point-of-usedistribution of a planarization fluid to a semiconductor waferplanarization machine. In one aspect, the regulating apparatus includesa planarization fluid storage tank with an acoustic fluid level sensorto detect the fluid level within the storage tank. The storage tank isconnected to a planarization fluid delivery line that deliversplanarization fluid to the storage tank through a flow control valve anddelivers a regulated flow of planarization fluid to a planarizationmachine through a flow measurement device. A gas supply system isconnected to the storage tank to provide system pressurization.Regulation of the planarization fluid flow from the regulating apparatusis achieved by a control system in which the flow measurement device andthe acoustic fluid level sensing capability comprise feedback elementsin a closed feedback system to independently control the pressure in thestorage tank and the amount of fluid admitted by the control valve. Inan alternate aspect, the fluid level sensor is comprised of an array ofcapacitive proximity sensors located outside the wall of the storagetank. In another alternate aspect, the fluid level sensor is replaced bya buoyant float that is partially buoyant in the planarization fluidthat is adapted to seat in the upper or lower ends of the storage tankwhen the storage tank is full or empty. Indication of the full and emptytank conditions are obtained from a differential pressure sensor whichis suitably located to sense differences in the storage tank pressurewhen the float is seated in either the upper or lower end of the storagetank. In still another aspect, two or more regulators may be joined in aparallel flow arrangement in order to achieve precise point-of-usemixing and flow rate control of multi-component planarization fluids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a prior art planarization fluiddistribution system.

FIG. 2 is a schematic view of a point-of-use planarization fluiddistribution system.

FIG. 3 is a schematic view of an embodiment of the planarization fluidflow regulator for use in a point-of-use system.

FIG. 4 is a schematic view of an alternative embodiment of theplanarization fluid flow regulator for use in a point-of-use system.

FIG. 5 is a schematic view of an alternative embodiment of theplanarization fluid flow regulator for use in a point-of-use system.

FIG. 6 is a schematic view of a point-of-use planarization fluiddistribution system using an embodiment of the planarization fluid flowregulator.

FIG. 7 is a diagram showing a flow for planarizing semiconductor waferswith a point-of-use planarization fluid system operating in theintermittent mode.

FIG. 8 is a diagram showing a flow for planarizing semiconductor waferswith a point-of-use planarization fluid system operating in thecontinuous mode.

In the drawings, like reference numbers identify similar elements orsteps. For ease in identifying the discussion of any particular element,the most significant digit in a reference number refers to the Figurenumber in which the element is first introduced (e.g., element 24 isfirst introduced and discussed with respect to FIG. 2).

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to an apparatus and methodof planarization fluid flow regulation that allows point-of-use mixingand distribution of planarization fluid to a planarization machine. Manyof the specific details of certain embodiments of the invention are setforth in the following description and in FIGS. 2 through 8 to provide athorough understanding of such embodiments. One skilled in the art willunderstand, however, that the present invention may have additionalembodiments, or that the present invention may be practiced withoutseveral of the details described in the following description. Forpurposes of the following description, the term planarization fluid mayrefer either to a planarization fluid with or without abrasiveparticles, or to a single component of a multi-component planarizationfluid that is combined with other components to comprise theplanarization fluid. In addition, terms of art such as “slurry” or“polishing slurry” or other similar terms are regarded as equivalent toplanarization fluid, as used herein. Moreover, specific dimensions andother physical characteristics related to different embodiments are notto be considered as limiting unless the claims expressly stateotherwise.

FIG. 2 is a schematic representation of a point-of-use planarizationfluid distribution system 20 that is capable of precise point-of-usemixing and flow rate control. With reference to FIG. 2, the componentsof a multi-component planarization fluid are stored in fluid dispensers21 a-21 c. For clarity of presentation, three fluid dispensers areshown, although more than three may be present, or only a singledispenser may be used. The fluid dispensers 21 a-21 c may be theoriginal containers in which the components of the planarization fluidwere shipped from the manufacturer, or they may be other containersadapted to retain planarization fluid components. Additionally, otherdispensers may be present to contain solutions that are used exclusivelyto recondition the pad, or to contain deionized water. The fluidcomponent streams 22 a-22 c originate at the fluid dispensers 21 a-21 cand are directed to a plurality of planarization fluid regulatingdevices 23 a-23 c as unregulated streams by a gravity feed system.Alternatively, fluid component streams 22 a-22 c may be pumped to theregulating devices 23 a-23 c by pumps (not shown). The regulatingdevices 23 a-23 c are capable of precisely regulating the fluidcomponent streams 22 a-22 c to provide a precisely selectedplanarization fluid flow rate at the regulated output streams 24 a-24 c.Associated with each of the regulating devices 23 a-23 c are fluidsensing systems 230 a-230 c that sense fluid motion and fluidaccumulation within the regulating devices 23 a-23 c, and fluid commandsystems 240 a-240 c that admit fluid to each of the regulating devices23 a-23 c and pressurize the fluid within the regulating devices 23 a-23c. The fluid sensing systems 230 a-230 c and fluid command systems 240a-240 c act cooperatively with a control system 200, which receivescontrol inputs from the fluid sensing systems 230 a-230 c through lines28 a-28 c and transmit control outputs to fluid command system 240 a-240c through lines 29 a-29 c to regulate the fluid component streams 22a-22 c. The reception and transmission of control inputs and outputsbetween the control system 200 and the fluid sensing systems 230 a-230 cand fluid command systems 240 a-240 c may utilize any available datatransmission method, and do not do not need to be physically connected.For example, the fluid sensing systems 230 a-230 c and fluid commandsystems 240 a-240 c may communicate with the control system 200 by meansof radio frequency energy. Additionally, the regulating devices 23 a-23c may be individually controlled by control systems which are integralto the regulating devices 23 a-23 c.

Subsequent to regulation by the devices 23 a-23 c, the regulated outputstreams 24 a-24 c are then directed to a mixing unit 25 for combinationbefore emerging as a regulated stream 26 for distribution onto theplanarizing pad of a semiconductor wafer planarization machine 27. Themixing unit 25 may comprise a simple mixing manifold for combining fluidstreams, or it may include active mixing devices such as mechanicalagitators. However, in order to attain a point-of-use system of limitedinternal volume, the internal volume of the mixing unit 25 should belimited, preferably, to a fraction of the combined internal volumes ofthe regulating devices 23 a-23 c.

A point-of-use planarization fluid distribution system that is capableof flow rate control of a single component planarization fluid isobtained when a single unregulated fluid stream is regulated. Forexample, referring to FIG. 2, single component regulation is obtainedwhen a single fluid dispenser 21 a is present, having a singleunregulated stream 22 a. Since no mixing of planarization fluidcomponents is required, unregulated stream 22 a is controlled to aspecified flow rate by the regulating device 23 a to achieve a regulatedoutput stream 24 a. As before, the regulated output stream 26 is thenultimately directed to a semiconductor wafer planarization machine 27for use in wafer planarizing. Since no mixing of components is requiredwhen a single component planarization is used, a mixing unit 25 isgenerally not required.

In order to achieve the advantages of point-of-use operability, it ispreferable that the fluid dispensers 21 a-21 c, the regulating devices23 a-23 c and the mixing unit 25 be located in close proximity to thesemiconductor wafer planarization machine 27 so that fluid volumesassociated with the delivery lines for fluid component streams 24 a-24 cand the regulated output stream 26 are minimized.

As previously described, an important aspect of the present inventionresides in the apparatus used to regulate planarization fluids at thepoint-of-use. In the foregoing discussion, various embodiments of aregulating device applicable to a point-of-use planarizationdistribution system will be described.

FIG. 3 is a schematic representation of an embodiment of a planarizationfluid regulating apparatus 30 according to the invention. As showntherein, a gas supply system 33 includes a pressure source 34 which maybe a high-pressure bottle or a centralized gas supply facility, tosupply pressurization to the system 30. Preferably, nitrogen is used asthe pressurization source 34, although a wide variety of gases and gasmixtures may be used. A pressure regulator 36 is connected to thepressure source 34 to reduce and regulate the source pressure tomoderate pressures compatible with the operation of the regulatingapparatus 30. The pressure regulator 36 is preferably a device that willpermit the outlet pressure from regulator 36 to be set and controlledbased upon a control input received from a control system 310 along line35. The control system 310 will be described in greater detail below.The control input to set the pressure at regulator 36 may be a digitalsignal, or alternatively, an analog voltage level. An example of apressure regulator that is responsive to an analog input voltage thatmay be used with this embodiment is the SMC Series ITV200 E-P Regulatormanufactured by SMC Pneumatics, Inc. of Indianapolis, Ind., althoughother alternatives exist.

Still referring to FIG. 3, the outlet pressure of the regulator 36 maybe optionally connected to a bubbler 38, which introduces moisture intothe regulated gas by bubbling the gas through a volume of deionizedwater 37. The introduction of moisture by means of the bubbler 38 may bepreferred in cases where the pressurized gas obtained from thepressurized source 34 has low moisture content. The regulated gas may beoptionally isolated from the system using a valve 336. This may bepreferred when the storage tank 32 and delivery line 31 are rinsed, aswill be discussed below.

Still referring to FIG. 3, a storage tank 32 that is capable of internalpressurization is used to contain a volume of a planarization fluid 328.The storage tank 32 also contains an internal gas space 329 that ispressurized by gas from the gas supply system 33, to impart fluidpressure to the planarization fluid 328. The storage tank 32 ispreferably comprised of TEFLON™, but other suitably non-contaminatingand non-reactive materials may be used. In addition, compositestructures may be used, such as a stainless steel tank that contains athin surface coating of TEFLON™, or other materials. The storage tank 32preferably has a conical top surface 332 and a conical bottom surface330 to allow the interior surface of the conical top surface 332, andthe interior surfaces of storage tank 32 to be rinsed more effectively.In addition, a conical bottom surface 330 is preferable because it willinhibit the formation of concentrations of abrasive particles or otherabrasive agglomerations. Other surface shapes are available asalternatives to the conical top surface 332 and the conical bottomsurface 330. For example, hemispherical or concave shapes may be used.

Rinsing of the interior surfaces of storage tank 32 may be required whenit becomes necessary to change to a planarization fluid of differentcomposition. Accordingly, a source of deionized water 335 may beprovided to the storage tank 32 through a valve 334. The gas supplysystem 33 may be isolated from the rinse water supply by closing valve336.

The storage tank 32 is further comprised of a fluid level sensor 322located on the conical top surface 332 to continuously monitor theplanarization fluid level 325 within the storage tank 32. In thisembodiment, the fluid level sensor 322 is an ultrasonic level detectorthat can directly sense the location of the fluid surface 325 withintank 32. An example of an ultrasonic level detector that may be used isthe Sonic OMNI-BEAM™ ultrasonic proximity detector manufactured byBanner Engineering Corporation of Minneapolis, Minn. However, otheralternative continuous level sensing devices and methods are available,which are interchangeable with the ultrasonic level detector. One suchalternative is a magnetostrictive level sensor, such as the LEVEL PLUS™magnetostrictive fluid level detector manufactured by MTS SystemsCorporation of Cary, N.C.

With reference still to FIG. 3, a planarization fluid delivery line 31is connected to the lower end of storage tank 32 at the bottom surfacelocation 331 by a fluid exchange line 391. The planarization fluiddelivery line 31 is preferably comprised of TEFLON™, although othernon-contaminating and non-reactive materials may be used. Theplanarization fluid delivery line 31 also includes a planarization fluidinlet 340 that is connected to an unregulated source of planarizationfluid. Fluid may be pumped into the planarization fluid delivery line 31by external pumping means (not shown), or the fluid may be introducedinto the delivery line 31 from a gravity feed system (not shown) or line31 may optionally have a pump 338 driven by motor 250 to transportplanarizing fluid through the line 31. In order to avoid contaminationof the planarizing fluid, the pump 338 is preferably a peristaltic pump,although other pumps could be used interchangeably. Flow admitted to theplanarization fluid delivery line 31 is controlled by a flow controlvalve 312 that has a continuously variable valve opening which is set bya valve actuator. Alternatively, the flow control valve 312 may provideonly an on/off capability. The flow control valve 312 is responsive to acontrol input received from the control system 310, which will bedescribed in greater detail below. The control input to set the valveposition at the flow control valve 312 may be either a digital signal,or a voltage level. An example of a TEFLON™-lined control valve that isresponsive to either a digital or an analog input that could be employedin this embodiment is the Tylan MDV motor driven valve manufactured bythe Millipore Corporation of Bedford, Mass.

Still referring to FIG. 3, a flow sensor 318 is included inplanarization fluid delivery line 31 to measure the rate ofplanarization fluid flow, which can transmit a control output to thecontrol system 310 along a line 319. Since the flow sensor 318 mustindicate the total amount of flow issuing from the regulating apparatus30, it must be located in the planarization fluid delivery line 31 at alocation downstream from the location where fluid exchange line 391joins line 31, and preferably, near the regulated fluid output location360. A flow sensor of the variable area type may be used as flow sensor318 in this embodiment, although other alternative flow sensortechnologies, such as ultrasonic flow sensors, thermal-pulse flowmeters,vortex-shedding flowmeters, or laminar element flowmeters may be used.An example of a TEFLON™-lined variable area flow sensor that could beemployed in this embodiment is the Model 4400 Flow meter manufactured byNT International, Inc. of Minneapolis, Minn.

Other flow conditioning components may be optionally included inplanarization fluid delivery line 31. For instance, a pressure dampingdevice 320 may be used to dampen the periodic pressure pulsations thatare generated by the pump 338, which may be objectionably pronouncedwhen a peristaltic pump is used. The device 320 is preferably anaccumulator having a closed vertical fluid column with a trapped gasspace filled with nitrogen. A plurality of accumulators 320 may be usedat different locations along the planarization fluid delivery line 31 asrequired. Additional dissipation of pressure pulsations may be attainedthrough the optional use of a flow restrictor 314 at a locationdownstream of the flow control valve 312 and an additional flowrestrictor 316 downstream of the flow sensor 318. Although two flowrestrictors are depicted in this embodiment, more than two may be used,and may be placed in other locations along the planarization fluiddelivery line 31 as required. Additionally, an optional flow shut offvalve 361 may be located near the fluid output location 360.

The control system for the planarization fluid regulating apparatus 30will now be described in detail. Referring to FIG. 3, the control system310 operates as a multi-input, multi-output (MIMO) closed loop controlsystem with the fluid level sensor 322 and the fluid flow sensor 318acting as feedback elements. The control system 310 provides controloutput signals to the pressure regulator 36 and the flow control valve312 so that the flow of planarization fluid that issues from theplanarization fluid outlet 260 is uniformly maintained. The controlalgorithm employed by the control system 310 should provide at least aproportional-integral (PI) capability, however, aproportional-integral-differential (PID) algorithm is preferred.Additional input and output means 311 are provided to allow theoperation of the flow regulating system 30 to be continuously monitoredand to allow the entry of commands. The control system 310 may be aprogrammable digital computer with stored instructions to execute thecontrol algorithm, with analog to digital (A/D) interfaces tocommunicate with the control devices, or it may be a self-containedprogrammable logic controller (PLC) capable of MIMO operation. Oneexample of a PLC that is capable of MIMO operation that uses a PIDalgorithm is the Keyence KV Series PLC manufactured by the KeyenceCorporation of America, Woodcliff Lake, N.J.

Turning now to FIG. 4, an alternative embodiment of the planarizationfluid regulating apparatus 40 is shown. In this embodiment, the locationof planarization fluid level 325 is determined by an array of proximitysensors 400 a-400 d that are located adjacent to the outer surface ofstorage tank 32. Although four proximity sensors are shown for clarityof presentation, a larger number of proximity sensors may be present.The proximity sensors 400 a-400 d are preferably capacitive proximitysensors that detect the presence of planarization fluid 328 by a changein capacitance. The sensors 400 a-400 d may be connected to amultiplexer 410 that sequentially interrogates the sensors 400 a-400 din order to provide an input control signal to the control system alongline 420. The use of capacitive proximity sensors has certain advantagesover the level sensing method used in the previous embodiment. Forexample, since a number of sensors are arrayed along the exteriorsurface of the storage tank 32, a single failure of a sensing element isunlikely to render the level sensing feedback element inoperative. Inaddition, since the capacitive proximity sensors 400 a-400 d can sensethe presence of planarization fluid through a nonmetallic container, nopenetrations through the wall of storage tank 32 are required. Althoughmany types of proximity sensors are available which may be usedsuccessfully with this embodiment, an example of a proximity sensor thatmay be used is the Type 53 capacitive proximity sensor manufactured bythe Cutler-Hammer Corporation of Milwaukee, Wis.

Turning now to FIG. 5, still another embodiment of the planarizationfluid regulating apparatus 50 is shown. This embodiment uses a float 52and a differential pressure transducer 54 to determine when the fluidlevel within the storage tank 32 has reached the maximum level. Thefloat 52 moves within the storage tank 32 as the planarization fluidlevel 325 rises or falls. A differential pressure transducer 54 has ahigh pressure sense port 520 in communication with the internal gasspace 329, and a low pressure sense port 530 connected to thepressurization gas source at a location 531. An indication of themaximum level 510 in tank 32 is generated when the level of fluid intank 32 rises to a maximum level 510, and the spherical float 52 seatsin the conical top surface 332. At this point, the high pressure senseport 520 is exposed to a higher pressure than the pressure establishedat low pressure sense port 530 so that the differential pressuretransducer 54 indicates a significantly non-zero and positive value forthe differential pressure. An input control signal is then transmittedto the control system 310 along a line 540 to indicate that theplanarization fluid level 325 has risen to the maximum level 510, andthat the flow control valve 312 should be commanded to close.Determination of the minimum level in tank 32 may only be inferred fromthe known flow rate measured by flow meter 318 and the elapsed time ofplanarization. Although the level sensing system described in thisembodiment may reflect only the full fluid level state, it issignificantly simpler than the previous embodiments, since it relies ona simple differential pressure transducer as the level sensing feedbackelement. Moreover, since the differential pressure transducer 54 onlyneeds to discriminate between a zero and a significantly non-zerodifferential pressure condition, a relatively low-cost transducer may beused.

Turning now to FIG. 6, an embodiment of the invention is shown thatpermits planarization fluid components of a multi-componentplanarization fluid to be accurately combined at the point of use. Asshown therein, two planarization fluid regulators, 601 and 602 arejointly operated in a parallel arrangement. Although this embodimentillustrates a system applicable to a two-component planarization fluid,additional regulators may be added in parallel for planarization fluidsthat are comprised of more than two components.

To achieve precise mixing of the planarization fluid components, a firstplanarization fluid component is supplied to regulator 601 from anunregulated source connected to planarization fluid inlet 340 a.Similarly, a second planarization fluid component is supplied toregulator 602 from an unregulated source connected to planarizationfluid inlet 340 b. The regulators 601 and 602 may be connected to acommon pressurization source 34, or alternatively, may be attached toseparate pressurization sources. The first and second planarizationfluid components are then independently regulated by regulators 601 and602 according to set points input to control systems 600 and 610. Theset points for the regulators 601 and 602 reflect the relativeproportions of the first and second components to be combined, and theflow rate of planarization fluid that must be delivered. As analternative, the control systems 600 and 610 may be combined into asingle control system to jointly control the regulators 601 and 602.

With the planarization fluid regulators 601 and 602 operating asdescribed above, a regulated output of the first planarization fluidcomponent is obtained at planarization fluid outlet 260 a, andsimilarly, a regulated output of the second component is obtained atoutlet 260 b. The regulated outputs at 260 a and 260 b are then combinedin a mixing tank 640 to achieve complete mixing of the first and secondplanarization fluid components to achieve fluid 656. The fluid 656 maythen be deposited on a planarization pad 654 of machine 650 through anoptional distribution device 642.

Numerous advantages are associated with the point of use mixingapparatus described above. For example, since the components of theplanarization fluid remain separated until they are combined in themixing unit 640 problems associated with planarization fluid degradationare minimized. Furthermore, since planarization fluid regulators 601 and602 have relatively small system volumes, the problems associated withlarge volume mixing and distribution systems is avoided.

A point-of-use fluid flow regulating system according to the disclosedembodiments may be configured to process semiconductor wafers in eitheran intermittent or a continuous mode. Briefly, when operating in theintermittent mode, the pressure supplied to the storage tank 32 is theonly element controlled by the control system 310, in response to afeedback signal from flow sensor 318. Flow control valve 312 remainsclosed while wafer planarization occurs so that no additional volume offluid is admitted to the storage tank 32. Operation of the point-of-usesystem 20 in the intermittent mode is advantageous when small batches ofwafers are to be processed. The continuous mode of operation uses boththe pressure supplied to the storage tank 32 and the flow control valve312 as elements controlled by the control system 310. Since the flowcontrol valve 312 may open during wafer planarization, the planarizationfluid volume 328 may be continuously replenished from fluid source 340while wafers are being planarized. These two modes of operation arediscussed more fully below.

Referring to FIG. 7, a flow diagram for planarizing semiconductor wafersusing a point-of-use planarization fluid distribution system operatingin the intermittent mode is shown. At step 1, unprocessed wafers areloaded onto a planarizing machine, and a set point command correspondingto a desired planarization fluid flow rate is input into the controlsystem 310 by an operator using the input-output means 311. The controlsystem 310 then transmits a control signal to the flow control valve 312along line 313 that opens valve 312, to admit a volume of planarizationfluid 328 to the storage tank 32 through planarization fluid deliveryline 31. The fluid level sensor 322 continuously monitors the volume offluid admitted, so that the storage tank 32 is filled to a maximumpermissible volume, or alternatively, to an operator-prescribed volumethat is less than a maximum permissible volume. After the planarizationfluid 328 has been admitted to the storage tank 32, flow control valve312 is commanded closed by control system 310. At step 2, regulation ofthe planarization fluid flow to the machine is established when thecontrol system 310 transmits a control signal to the pressure regulator36 to pressurize the internal gas space 329 in storage tank 32. Inresponse, planarization fluid enters the planarization fluid deliveryline 31 and proceeds along delivery line 31 to the fluid flow sensor318, which monitors the rate of fluid flow delivered by theplanarization fluid regulating apparatus 30. In order to maintain theprescribed rate of fluid flow emanating from the planarization fluidoutlet 260, the gas pressure in the internal gas space 329 iscontinuously adjusted by the control system 310 in response to flow rateinformation received from flow sensor 318. At step 3, the wafers areplanarized in the conventional manner while a regulated flow ofplanarization fluid is deposited on the planarization pad. As the waferplanarization proceeds, however, fluid is continuously removed from thetank. Ordinarily, the wafer planarization endpoint will be reachedbefore the fluid is depleted, since the storage tank 32 is generallysized to accommodate a volume of planarization fluid that is sufficientto complete the wafer planarization. However, if an insufficient amountof fluid remains in the storage tank 32, the low fluid level conditionwill be detected by the fluid level sensor 322. Alternatively, the fluidlevel may be inferred from the known flow rate as measured by flowsensor 318 multiplied by the elapsed time since fluid distributionstarted. If a low fluid level is detected, an appropriate alert signalmay be sent to the control system 310, which, in turn, will provide anappropriate advisory message to an operator, as shown in step 4. Step 5concludes the process with the unloading of processed wafers from themachine. At this step, the process may be terminated, or repeated byreturning to step 1.

With reference now to FIG. 8, a flow diagram for planarizingsemiconductor wafers using a point-of-use planarization fluiddistribution system operating in the continuous mode is shown. At step1, unprocessed wafers are loaded onto the planarization machine and aset point command is input into the control system 310 using theinput-output means 311. The control system 310 then transmits a controlsignal to the flow control valve 312 to open, admitting a volume ofplanarization fluid 328 to the storage tank 32 through planarizationfluid delivery line 31. When the maximum level of planarization fluidhas been attained in storage tank 32, the flow control valve 312 iscommanded to close. At step 2, the control system 310 transmits acontrol signal to the pressure regulator 36 to pressurize the internalgas space 329 in storage tank 32, to apply a pressure to theplanarization fluid 328. In response, planarization fluid enters theplanarization fluid delivery line 31 and proceeds along delivery line 31to the fluid flow sensor 318 to establish a regulated flow ofplanarization fluid from the outlet 360. At step 3, semiconductor wafersare planarized in the conventional manner in the presence of thecontinuously regulated flow. As the wafer planarization process consumesthe fluid in storage tank 32, the control system 310 continuouslyadjusts the gas pressure in the internal gas space 329 to maintain theprescribed flow rate of planarization fluid while monitoring theposition of planarization fluid level 325 in storage tank 32. Inresponse to the fluid level detected within storage tank 32, the controlsystem 310 modulates the position of flow control valve 312 to ensurethat a sufficient amount of fluid is admitted at inlet 340 to keep thestorage tank 32 sufficiently filled with planarization fluid. As aresult, the wafers may be continuously planarized until an endpoint isreached. At step 4 of the process, the regulated flow of fluid isinterrupted, either by the optional valve 361 attached to the outlet360, or by releasing the pressure in the storage tank 32 and closing theflow control valve 312, but preferably, valve 361 is used, since wasteof the planarization fluid would be minimized. The processed wafers maynow be unloaded from the machine. At step 5, unprocessed wafers areloaded onto the machine, and the regulated flow is reestablished at step2 to planarize the wafers. When the regulated flow is reestablished, thecontrol system 310 may either impose the regulated flow rate used forthe previous planarization cycle, or alternatively, an updated flow ratemay be established. An updated planarization flow rate is particularlyadvantageous, since it may be used to compensate for changes that occurto the wafer planarization pad as successive batches of wafers areplanarized.

The above description of illustrated embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed. While specific embodiments of, and examples of, the inventionare described in the foregoing for illustrative purposes, variousequivalent modifications are possible within the scope the invention, asthose skilled in the relevant art will recognize. Moreover, the variousembodiments described above can be combined to provide furtherembodiments. Accordingly, the invention is not limited by thedisclosure, but instead the scope of the invention is to be determinedentirely by the following claims.

What is claimed is:
 1. A system for regulating a flow of a planarizationfluid, comprising: a fluid regulating device configured to receive theplanarization fluid and to release a regulated flow of the fluid to asemiconductor wafer planarization machine, the fluid regulating devicefurther including a fluid sensing system configured to sense a fluidmotion, and a fluid command system configured to control the regulatedflow of fluid from the regulating device; and a control system coupledto the fluid sensing system and the fluid command system and operable toreceive control inputs from the fluid sensing system and operable totransmit control outputs to the fluid command system.
 2. The system ofclaim 1, wherein the system further comprises an internal volume foraccumulating the planarization fluid, and the sensing system furthercomprises a flow rate sensor to sense a fluid flow leaving the internalvolume and a fluid level sensor to sense a surface level of the fluidaccumulated within the internal volume.
 3. The system of claim 2,wherein the fluid command system further comprises a flow control deviceto interruptably admit fluid to the internal volume, and apressurization device to apply a pressure to the fluid accumulatedwithin the volume.
 4. The system of claim 3, wherein the flow controldevice comprises a flow control valve.
 5. The system of claim 3, whereinthe pressurization device comprises a source of a pressurized fluidcoupled to a pressure regulator.
 6. The system of claim 5, wherein thepressure regulator comprises a regulator responsive to an input signal.7. The system of claim 2, wherein the flow rate sensor comprises avariable area flow sensor.
 8. The system of claim 2, wherein the fluidlevel sensor comprises an acoustic fluid level sensor.
 9. A system forthe point-of-use mixing of a planarization fluid having more than asingle component, comprising: fluid supply dispensers each containing arespective component of the fluid; a fluid regulating device coupled toeach dispenser to receive an input stream of a fluid component from arespective one of the fluid supply dispensers and to release an outputstream of the respective fluid component, each fluid regulating devicehaving a having a fluid sensing system to sense a fluid motion withinthe regulating device and a fluid command system to control a fluid flowfrom the regulating device; a control system coupled to the fluidsensing systems and fluid command systems to receive control inputs fromthe fluid sensing systems and to transmit control outputs to the fluidcommand systems to control the fluid component output streams; and amixing unit to receive the fluid component output streams.
 10. Thesystem of claim 9, wherein the fluid regulating device further comprisesan internal volume structured to accumulate a fluid volume, and furtherwherein each of the fluid sensing systems comprises a flow rate sensorto sense a flow rate of fluid leaving the internal volume and a fluidlevel sensor device configured to sense a fluid surface level of thefluid accumulated within the internal volume.
 11. The system of claim 10wherein each of the fluid command systems comprises a device configuredto interruptably admit the fluid input stream from the fluid supplydispenser to the internal volume, and a pressurization device to apply apressure to the fluid accumulated within the volume to propel the fluidcontained therein as an output stream.
 12. The system of claim 11wherein the pressurization device comprises a source of pressurizedfluid in fluid communication with a pressure regulator.
 13. The systemof claim 12 wherein the pressure regulator is further comprised of aregulator responsive to an input signal.
 14. The system of claim 10wherein the flow rate sensor is further comprised of a variable areaflow sensor.
 15. The system according to claim 10 wherein the fluidlevel sensor is an acoustic fluid level sensor.
 16. The system of claim9 wherein at least one of the fluid supply dispensers comprises adispenser structured to contain a planarization pad conditioningsolution.
 17. The system of claim 9 wherein at least one of the fluidsupply dispensers comprises a dispenser structured to contain deionizedwater.
 18. An apparatus for regulating the flow of a planarizationfluid, comprising: a dispenser containing a planarization fluid; astorage tank structured to contain a volume of the planarization fluidand having a fluid level sensing device responsive to a fluid level ofthe planarization fluid contained within the tank and capable oftransmitting a control output; a pressurization system coupled to thestorage tank to pressurize the volume contained in the storage tank, thepressurization system being responsive to a control input; a fluiddelivery line coupled to the dispenser to receive the planarizationfluid, the fluid delivery line further including a flow control deviceresponsive to a control input and a flow measurement device configuredto transmit a control output, the planarization fluid delivery linebeing in fluid communication with the storage tank at a location betweenthe flow control device and the flow measurement device to exchangeplanarization fluid with the storage tank; and a control system thatreceives the control inputs from the fluid level sensing device and theflow measurement device and transmits control outputs to thepressurization system and the flow control device to regulate the flowof planarization fluid to a predetermined flow rate.
 19. The apparatusof claim 18, wherein the pressurization system is comprised of a sourceof pressurized fluid in fluid communication with a pressure regulatorconfigured to regulate the source of pressurized fluid to a prescribedlevel.
 20. The apparatus of claim 19, wherein the pressurization systemfurther comprises a bubbler connected to and positioned between thesource of pressurized fluid and the pressure regulator to introducemoisture into the pressurized fluid.
 21. The apparatus of claim 19,wherein the source of pressurized fluid is pressurized nitrogen.
 22. Theapparatus of claim 18, wherein the fluid delivery line is comprised of aplurality of flow restrictors.
 23. The apparatus of claim 18 whereinflow control device comprises a flow control valve to control theadmission of fluid into the fluid delivery line.
 24. The apparatus ofclaim 23, wherein the fluid delivery line further comprises a pumplocated between the flow control valve and the fluid supply dispenser.25. The apparatus of claim 24, wherein the fluid delivery line furthercomprises of a first flow restrictor positioned between the pump and thefluid exchange line connection.
 26. The apparatus of claim 25, whereinthe fluid delivery line is further comprised of an accumulatorpositioned between the flow control valve and the first flow restrictor.27. The apparatus of claim 24, wherein the fluid delivery line furthercomprises a second flow restrictor located between the flow measurementdevice and a mixing unit.
 28. The apparatus of claim 18, wherein thestorage tank is further comprised of a fluid exchange line connected tothe fluid delivery line at a location between the flow control valve andthe flow measurement device to transport fluid to and from the storagetank.
 29. The apparatus of claim 18 wherein the fluid level sensingdevice further comprises an acoustic level sensor.
 30. The apparatus ofclaim 18, wherein the fluid level sensing device further comprises amagnetostrictive level sensor.
 31. The apparatus of claim 18, whereinthe fluid level sensing device further comprises a plurality ofcapacitive proximity sensors located adjacent to the storage tank. 32.The apparatus of claim 18, wherein the flow measurement device furthercomprises a variable area flow sensor.
 33. The apparatus of claim 18,wherein the flow measurement device further comprises an ultrasonic flowsensor.
 34. The apparatus of claim 18, wherein the flow measurementdevice further comprises a vortex-shedding flow sensor.
 35. Theapparatus of claim 18, wherein the flow measurement device furthercomprises a laminar cell flow sensor.
 36. The apparatus of claim 18,wherein the control system further comprises a dedicated logiccontroller with a stored control algorithm.
 37. The apparatus of claim30, wherein the stored control algorithm comprises at leastproportional-integral control.
 38. The apparatus of claim 30, whereinthe stored control algorithm comprisesproportional-integral-differential control.
 39. The apparatus of claim18, wherein the control system further comprises a programmable digitalcomputer with a stored control algorithm.
 40. The apparatus of claim 18,wherein the control system further comprises a user interface whichaccepts operating instructions from a user, and displays operating datato the user.
 41. The apparatus of claim 18, wherein the storage tankfurther comprises a storage tank comprised of TEFLON.
 42. The apparatusof claim 22, wherein the storage tank is further comprised of aninternal volume of approximately about 1.5 liters.
 43. The apparatus ofclaim 22, wherein the storage tank is further comprised of an upper endwith a conical surface sloping upward to form a centrally located firstopening for receiving a pressurized fluid, and a lower end comprised ofa conical surface sloping downward to form a centrally located secondopening for transferring planarization fluid, the upper end and lowerend further having a substantially vertical side wall between andadjacent to the upper and lower end to form a vessel further having aninterior volume.
 44. The apparatus of claim 43, wherein the fluid levelsensing device is further comprised of a buoyant float adapted to be atleast partially buoyant in a planarization fluid, and a differentialpressure sensor with a first sense port in pressure communication withthe pressurization source at the first opening, and a second sense portin pressure communication with the interior space, and further whereinthe buoyant float is comprised of a spherical float that is structuredto sealably restrict the first opening and the second opening.